Air and Atmosphere

The most abundant gas present in air is:

1. Nitrogen
2. Oxygen
3. Carbon dioxide
4. Water vapour

Answer

Nitrogen

Reason — The most abundant gas present in air is Nitrogen. Nitrogen makes up about 78% of air.
Oxygen is about 21% of air. Important for respiration but not the most abundant. Carbon dioxide is present in a very small amount (0.04%). Water vapour amount varies (0-4%).

The gas used to prepare baking soda is :

1. Nitrogen oxide
2. Carbon dioxide
3. Sulphur dioxide
4. Nitrogen dioxide

Answer

Carbon dioxide

Reason — The gas used to prepare baking soda is carbon dioxide.
Nitrogen oxide is not used in preparing baking soda. Sulphur dioxide is mainly used as a preservative and bleaching agent, not for baking soda. Nitrogen dioxide is a toxic gas, not used in baking soda preparation.

Which of the following is the main constituent of proteins ?

1. Methane
2. Sulphur dioxide
3. Nitrogen
4. Oxygen

Answer

Nitrogen

Reason — The main constituent of proteins is nitrogen. Proteins are made up of amino acids, and amino acids contain nitrogen in their amino group. This nitrogen is a key element in the structure of proteins.
Methane is a hydrocarbon (CH₄) and not related to protein composition. Sulphur dioxide is a pollutant gas, not a component of proteins. Oxygen is present in proteins but not the main defining element.

The presence of which substance in air causes pollution ?

1. Smoke
2. Oxides of sulphur
3. Chlorofluorocarbons
4. All of the above

Answer

All of the above

Reason — All the given substances cause air pollution.

• Smoke — contains particulate matter and unburnt hydrocarbons that pollute the air.
• Oxides of sulphur (SO₂, SO₃) — cause acid rain and respiratory problems.
• Chlorofluorocarbons (CFCs) — damage the ozone layer and contribute to global warming.

The gas which protects the earth from ultraviolet rays is:

1. oxygen
2. carbon dioxide
3. inert gases
4. ozone

Answer

ozone

Reason — The ozone layer in the Earth’s stratosphere absorbs most of the Sun’s harmful ultraviolet (UV) radiation, preventing it from reaching the surface and protecting living organisms.
Oxygen is essential for respiration but does not protect the earth from ultraviolet (UV) rays. Carbon dioxide helps in photosynthesis and the greenhouse effect, not UV protection. Inert gases like helium and neon are unreactive and do not protect from UV rays.

Name the following:

(a) The process by which atmospheric nitrogen is converted into nitrate and nitrite salts and gets fixed in the soil.

(b) Substances which burn in air to produce a large amount of heat and light energy.

(c) The amount of moisture present in the air.

(d) Tiny droplets of water which remain suspended in air.

(e) Harmful substances present in air which cause pollution.

Answer

(a) Nitrogen fixation

(b) Fuels

(c) Humidity

(d) Mist

(e) Air pollutants

Define the following terms:

(a) Pollutants

(b) Acid rain

(c) Global warming

(d) Smog

Answer

(a) Pollutants are the undesirable substances present in air that are harmful to plants and animals.

(b) A rain which is acidic in nature is called acid rain. Gases like sulphur dioxide and nitrogen dioxide react with water present in air to form sulphurous acid, sulphuric acid, nitrous acid and nitric acid. These acids come down to the earth along with rainwater as acid rain.

(c) An increase in the percentage of carbon dioxide, methane, nitrous oxide and chlorofluorocarbons traps the heat causing the temperature of the earth and its surroundings to rise. This is known as global warming.

(d) Oxides of nitrogen form a mixture of smoke and fog known as smog, which affects our eyes too.

Give one use for each of the following inert gases:

(a) Argon

(b) Helium

(c) Neon

(d) Radon

(e) Krypton

(f) Xenon

Answer

(a) Argon is used to fill electric bulbs to improve their quality and life.

(b) Helium is used for filling up weather observation balloons.

(c) Neon is used in making advertising signboards and tube-lights.

(d) Radon is used for cancer treatment.

(e) Krypton is used for photography.

(f) Xenon is used for photography.

Answer the questions put against each of the following constituents of air:

(a) Nitrogen : Explain its significance for plants and animals.

(b) Oxygen : What is the percentage proportion of oxygen in air? Why is oxygen called active air?

(c) Carbon dioxide : "Although carbon dioxide plays no role in respiration, all life would come to an end if there is no carbon dioxide in air." Support this statement with relevant facts.

(d) Water vapour : Explain its role in modifying the earth's climate.

Answer

(a) Nitrogen is an important constituent of proteins, which are essential for the growth and development of plants and animals. Plants use nitrogen to prepare proteins, while animals and human beings obtain proteins from plants.

(b) Oxygen constitutes about 21% of air by volume. Oxygen is called active air because it supports combustion and life.

(c) "Although carbon dioxide plays no role in respiration, all life would come to an end if there is no carbon dioxide in air." This statement is justified by following two facts:

1. Carbon dioxide is essential for the process of photosynthesis, as green plants require it to prepare food. All animals, including human beings, depend upon plants for food.
2. It minimises heat loss by radiation by reflecting heat back to the earth's surface, particularly at night, thus balancing the temperature of the earth.

(d) The presence of water vapour in air causes rain. At low temperatures water vapour condenses to form snow, sleet, mist, frost, hail, dew, fog, etc. In this way water vapour plays an important role in modifying earth's climate.

"Air is a mixture." Support this statement citing at least three evidences.

Answer

"Air is a mixture." This statement is supported by following three evidences:

1. The composition of air is not fixed. It varies from place to place and from time to time.
2. The components of air retain their individual properties.
3. No energy exchange occurs when the components of air are mixed with each other.

What is air pollution? Give one harmful effect each of sulphur dioxide, nitrogen dioxide and hydrogen sulphide present in the air?

Answer

When air contains undesirable substances that are harmful to plants and animals, the air is said to be polluted. The harmful effects of sulphur dioxide, nitrogen dioxide and hydrogen sulphide present in the air are:

1. Sulphur dioxide is a highly poisonous gas which causes serious respiratory problems.
2. Nitrogen dioxide forms a mixture of smoke and fog known as smog, which affects our eyes.
3. Hydrogen sulphide is highly obnoxious and causes headache.

(a) What are the causes of air pollution?

(b) Suggest five measures to prevent air pollution.

Answer

(a) The causes of air pollution are:

1. Natural processes like volcanic eruptions and crop pollination.
2. Burning of fuels and fibres.
3. Deforestation.
4. Emission of harmful gases from vehicles.
5. Use of chemicals like fertilizers, insecticides and pesticides in agricultural activities.
6. Increase in number of industries.
7. Use of chemical weapons in war.

(b) The five measures to prevent air pollution are:

1. By growing more plants and trees.
2. By installing tall chimneys with filters in factories and power plants.
3. By using properly maintained internal combustion engines in vehicles for complete and efficient burning of fuel.
4. By installing catalytic convertors in vehicles.
5. By avoiding the burning of paper, garbage, dry leaves and other waste materials.

A fuel that releases the least amount of pollutants in the air is:

1. Sulphur dioxide
2. Chlorofluorocarbons
3. Smoke
4. CNG

Answer

CNG

Reason — CNG is Compressed Natural Gas which is a clean burning fuel and causes very less pollution.
Sulphur dioxide is a pollutant gas, not a fuel. Chlorofluorocarbons are harmful chemicals, not fuels. Smoke is a result of burning fuels and causes pollution.

The natural way of adding oxygen to air which involves green plants is called:

1. Photosynthesis
2. Respiration
3. Burning
4. Dissolution

Answer

Photosynthesis

Reason — Oxygen is naturally renewed in air by the process of photosynthesis. Plants use carbon dioxide and water and in presence of sunlight produce glucose and oxygen gas. This oxygen gas is released into air.
Respiration is break down food using oxygen to release energy, producing carbon dioxide and water. Burning consumes oxygen instead of adding it. Dissolution is substances dissolving in liquids, not adding oxygen to air.

Which one of the following is most likely to be corroded?

1. a stainless steel cup-board
2. a galvanised iron bucket
3. an iron hammer
4. a tin-plated iron box

Answer

an iron hammer

Reason — Iron corrodes easily in the presence of air and moisture. Since an iron hammer is not protected by any coating, it is most likely to get corroded.
Stainless steel resists corrosion, while galvanised iron and tin-plated iron are protected by coatings of zinc and tin respectively.

The substance which produces oxygen gas when heated is :

1. Potassium chloride
2. Potassium chlorate
3. Potassium hydroxide
4. Potassium carbonate

Answer

Potassium chlorate

Reason — When potassium chlorate (KClO₃) is heated, it decomposes to produce potassium chloride (KCl) and oxygen gas (O₂).

$\text{Potassium chlorate} \xrightarrow[Manganese;dioxide]{heating} \text{Potassium chloride} + \text{Oxygen(g)}$

$\text{2KClO}_{3} \xrightarrow[\text{MnO}_{2}]{heating} \text{2KCl} + \text{3O}_{2}$

The process by which oxidation of food takes place in our body is called :

1. Combustion
2. Respiration
3. Photosynthesis
4. Excretion

Answer

Respiration

Reason — In our body, respiration is the process by which the oxidation of food (like glucose) takes place to release energy, carbon dioxide, and water.

Glucose + Oxygen ⟶ Carbon dioxide + Water vapour + Energy

Combustion is the process of burning substances in oxygen to produce heat and light. This does not occur inside our body for food. Photosynthesis is the process by which green plants make food using sunlight, carbon dioxide, and water. It does not occur in humans. Excretion is the removal of waste substances from the body, not related to oxidation of food.

A catalyst used in the preparation of oxygen is :

1. Manganese oxide
2. Potassium permanganate
3. Manganese dioxide
4. Manganese (II) chloride

Answer

Manganese dioxide

Reason — When potassium chlorate (KClO₃) is heated to prepare oxygen gas, manganese dioxide (MnO₂) is used as a catalyst. It speeds up the decomposition reaction without being consumed in it.

$\text{2KClO}_{3} \xrightarrow[\text{MnO}_{2}]{heating} \text{2KCl} + \text{3O}_{2}$

Assertion (A): Air is invisible and transparent.

Reason (R): Air is a mixture of colourless gases.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Reason — The Assertion is true because air cannot be seen through our eyes and allows light to pass through it, so it is invisible and transparent.

The Reason is also true because air is a mixture of colourless gases such as nitrogen, oxygen, carbon dioxide and inert gases.

Since these gases are colourless and transparent, air also appears invisible and transparent. Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Assertion (A): During the process of photosynthesis, plants take in carbon dioxide and give out oxygen.

Reason (R): Carbon dioxide dissolved in water adds taste to water.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Reason — A is true as plants take in carbon dioxide and release oxygen during photosynthesis.
R is true as carbon dioxide dissolved in water adds slightly sour or tangy taste.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

Assertion (A): The component of air which is required by living organisms is only oxygen.

Reason (R): Oxygen is essential for respiration and combustion.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Reason — A is false as living organisms need not only oxygen but also other components like carbon dioxide (for plants) and nitrogen (indirectly).
R is true as oxygen is essential for respiration and supports combustion.

Therefore, A is false but R is true is the correct option.

Assertion (A): Nitrogen is an important constituent of proteins which is necessary for growth in humans, plants and animals.

Reason (R): Nitrogen supports combustion.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Reason — A is true as nitrogen is a key component of proteins needed for growth.
R is false as nitrogen does not support combustion; it actually helps in preventing it.

Therefore, A is true but R is false is the correct option.

Fill in the blanks:

(a) ............... is the most abundant inert gas present in air.

(b) Oxides of sulphur and nitrogen combine with rain water to form ............... and ............... which cause ............... .

(c) ............... and ............... are the most common air pollutants.

(d) ............... discovered the oxygen gas.

(e) Oxygen occupies about ............... of air by volume.

Answer

(a) Argon

(b) sulphuric acid and nitric acid, acid rain

(c) smoke, dust

(d) Joseph Priestley

(e) 21%

Write 'true' or 'false' for the following statements :

(a) When magnesium is burnt, it gains weight.

(b) Oxygen reacts with metals to produce an acidic oxide.

(c) Chlorofluorocarbons cause ozone depletion.

(d) Oxygen changes moist red litmus paper to blue.

(e) Symbiotic bacteria living in the root nodules of leguminous plants absorb nitrogen directly from the air.

Answer

(a) True

(b) False
Correct Statement — Oxygen reacts with metals to produce basic oxides.

(c) True

(d) False
Correct Statement — Oxygen does not change moist red litmus paper to blue, or vice versa.

(e) True

Match the following:

Answer

Name:

(a) The most abundant element in the earth's crust.

(b) A chemical called oxygenated water.

(c) A metal highly resistant to rusting.

(d) A mixture of oxygen and carbon dioxide used for artificial respiration.

(e) Two substances from which oxygen can be obtained on a large scale.

(f) An oxide and a carbonate containing oxygen.

(g) Two substances which undergo rapid oxidation.

(h) A gas used in fire extinguishers.

Answer

(a) Oxygen

(b) Hydrogen peroxide (H₂O₂)

(c) Platinum

(d) Carbogen

(e) Air, water

(f) Sodium oxide (Na₂O), Calcium carbonate (CaCO₃)

(g) Sodium, carbon

(h) Carbon dioxide

Give two examples of the following:

(a) Greenhouse gases.

(b) Substances which cause air pollution.

(c) Gases responsible for acid rain.

(d) Gases which increase the process of rusting.

(e) Fuels which produce energy without leaving any residue.

Answer

(a) Carbon dioxide, methane

(b) Smoke, oxides of carbon

(c) Sulphur dioxide, nitrogen dioxide

(d) CO₂, SO₂

(e) CNG, LPG

Define :

(a) Oxidation

(b) Catalyst

(c) Respiration

(d) Ignition temperature

Answer

(a) Oxidation — Oxygen is chemically an active gaseous element. It supports burning, therefore metals, non-metals and a large number of compounds react with oxygen to produce new products which essentially contain oxygen in them along with other elements. The new compounds so formed are called oxides and the process is called oxidation.

(b) Catalyst — A catalyst is a substance that increases or decreases the rate of a chemical reaction without itself undergoing any chemical change.

(c) Respiration — Respiration is a natural chemical process in which oxygen in the air is used by living organisms to oxidise their food and to produce energy along with carbon dioxide and water vapour.

Glucose + Oxygen ⟶ Carbon dioxide + Water vapour + Energy

(d) Ignition temperature — The minimum temperature to which a substance must be heated to make it burn is called its ignition temperature.

What do you observe when the following substances are heated and then tested with moist blue and red litmus paper?

(a) Sulphur

(b) Phosphorus

(c) Calcium

(d) Magnesium

Answer

(a) Sulphur burns with a bright bluish flame, giving pungent smell of sulphur dioxide and it turns moist blue litmus paper red.

(b) Phosphorus burns with a dazzling flame producing dense white fumes of phosphorus pentoxide and it turns moist blue litmus paper red.

(c) Calcium burns with a bright brick red flame forming a white solid and it turns moist red litmus paper blue.

(d) Magnesium burns with a bright dazzling light forming white powder of magnesium oxide and it turns moist red litmus paper blue.

What are the two types of oxidation processes? Among these, which one produces a large amount of heat and light energy ? Which type of oxidation are respiration and rusting reactions ?

Answer

There are two types of oxidation processes:

1. Fast oxidation — When a substance combines with oxygen with the release of a large amount of heat and light energy, the process is called fast oxidation.
2. Slow oxidation — When a substance combines with oxygen slowly over a period of time with the release of a very small amount of heat energy, it is called slow oxidation.

• Fast oxidation process produces a large amount of heat and light energy.

• Respiration and rusting are examples of Slow Oxidation processes.

What are oxides? Give two examples of metallic and non-metallic oxides each.

Answer

Oxides are binary compounds formed by the chemical combination of a substance (metal or non-metal) with oxygen.

Two examples of metallic oxides are sodium oxide (Na₂O), calcium oxide (CaO).

Two examples of non-metallic oxides are sulphur dioxide (SO₂), carbon dioxide (CO₂).

State two differences between: Rusting and Burning.

Answer

State two differences between: Respiration and Burning.

Answer

(a) Taking hydrogen peroxide, how would you prepare oxygen in the laboratory?

(b) What is the role of manganese dioxide in the preparation of oxygen?

(c) Write the word equation for the above chemical reaction.

(d) Why is hydrogen peroxide preferred in the preparation of oxygen gas?

(e) Why is oxygen collected by downward displacement of water?

(f) What happens when a glowing splinter is introduced in a jar containing oxygen?

(g) What happens when oxygen gas is passed through alkaline pyrogallol solution?

Answer

(a) Oxygen is prepared from hydrogen peroxide in the laboratory in the following way:

1. A small quantity of manganese dioxide is taken in a flat bottom flask.
2. Hydrogen peroxide is added drop by drop into the flask with the help of a thistle funnel as shown in the below figure.

3. Manganese dioxide decomposes hydrogen peroxide very fast and liberates oxygen which passes through the delivery tube and bubbles out through the water in the trough.
4. A gas jar filled with water is inverted over a beehive shelf. Oxygen is collected in the jar by downward displacement of water.

(b) Manganese dioxide acts as a catalyst. It decomposes hydrogen peroxide very fast and liberates oxygen.

(c) Hydrogen peroxide $\xrightarrow{\text{Manganese dioxide}}$ Water + Oxygen

(d) Hydrogen peroxide is preferred in the preparation of oxygen gas because:

1. No heating is required.
2. The rate of evolution of oxygen is moderate and therefore under control.
3. Hydrogen peroxide is a safe chemical.

(e) Oxygen is collected by downward displacement of water because:

1. Oxygen is slightly heavier than air, so it cannot be collected over air.
2. Oxygen is only slightly soluble in water, i.e., 0.7% only. So it can be collected over water without fear of excessive dissolution.

(f) When a glowing splinter is introduced in a jar containing oxygen, the glowing splinter rekindles, but the gas does not catch fire.

(g) When oxygen gas is passed through alkaline pyrogallol solution, it turns brown.

Give reasons:

Why is potassium chlorate not preferred for the laboratory preparation of oxygen gas?

Answer

Potassium chlorate is heated to a high temperature to obtain oxygen gas, which may cause:

1. cracking of glass apparatus being used.
2. accidents and burn injuries. Thus, potassium chlorate is not preferred for laboratory preparation of oxygen gas.

Give reasons:

Carbon dioxide gas is used in fire extinguisher.

Answer

Carbon dioxide gas is used in fire extinguishers as it does not support burning. Carbon dioxide is heavier than air, hence it settles like a blanket over the fire and cuts off its oxygen supply due to which the fire gets extinguished.

(a) Give four uses of oxygen.

(b) How is oxygen naturally renewed in air ?

(c) Write the missing part/word in the following figure representing the oxygen cycle.

Answer

(a) The four uses of oxygen are:

1. Respiration
2. Burning
3. Industrial uses like welding and cutting metals, removing impurities.
4. In medicines like oxygen cylinders for artificial respiration in patients suffering from breathing problems, a mixture of oxygen and nitrous oxide as local anaesthesia.

(b) Oxygen is naturally renewed in air by the process of photosynthesis. Plants use carbon dioxide and water in the presence of sunlight to produce glucose and oxygen gas. This oxygen gas is released into air.

(c) The missing part/word in the given figure :

Sophia along with her classmates and teacher went on a school trip to Agra. She got an opportunity to visit Taj Mahal, the famous monument, made up of white marble. Sophia had heard a lot about its beauty but she was surprised to see the dull and pale yellow appearance of Taj Mahal.
Her teacher explained that it is due to the various factories in Agra, which emit harmful gases of sulphur and nitrogen in the atmosphere. These gases produce acid rain which has adverse effects on human health, animals, crops, buildings, etc.

(a) Name the gases which form the acid rain.

(b) What are the acids formed in the acid rain ?

(c) Give two balanced chemical equations to show the formation of acid rain.

(d) Why is Taj Mahal slowly turning yellow from white ?

(e) How can renewable sources of energy prevent air pollution ?

Answer

(a) The gases which form the acid rain are sulphur dioxide and nitrogen dioxide.

(b) The acids formed in the acid rain are sulphurous acid, sulphuric acid, nitrous acid and nitric acid.

(c) Two balanced chemical equations to show the formation of acid rain are :

SO₂ + H₂O ⟶ H₂SO₃

2NO₂ + H₂O ⟶ HNO₂ + HNO₃

(d) The Taj Mahal is slowly turning yellow from white because it is made of marble, i.e. calcium carbonate. Acid rain reacts with marble and forms powdery substances. Hence, the marble surface loses its shine and slowly changes colour from white to yellow.

(e) Renewable sources of energy such as solar energy, wind energy and hydroelectricity reduce the use of fossil fuels like coal and petroleum. As no fuel is burnt, less smoke and fewer harmful gases such as sulphur dioxide and nitrogen dioxide are released into the air. Thus, renewable sources of energy help in preventing air pollution and acid rain.